U.S. patent number 5,146,783 [Application Number 07/634,226] was granted by the patent office on 1992-09-15 for liquid container hydrostatic level gauge.
This patent grant is currently assigned to Robert Bosch GmbH. Invention is credited to Anton Dukart, Walter Jansche, Erich Zabler.
United States Patent |
5,146,783 |
Jansche , et al. |
September 15, 1992 |
Liquid container hydrostatic level gauge
Abstract
A liquid container hydrostatic level gauge, in particular a fuel
tank level indicator, which operates with a differential pressure
sensor, a zero point calibration related to the height of the fuel
is performed upon the beginning of measurement, in order to
dispense with a requirement for temperature independence and
long-term stability of the zero point of the differential pressure
sensor. A meter tube is disposed in the container interior,
extending from above the maximum liquid level to as far as the
container bottom with an opening for receiving fuel within the
tank. The differential pressure sensor is connected by one meter
input to the upper end of the meter tube and by its other meter
input is exposed to the air pressure acting upon the surface of the
liquid. An air pump is connected to the upper end of the meter tube
between the upper level of the fuel and the input to the pressure
sensor. At the beginning of measurement, the meter tube is flooded,
and an output value (U.sub.o) of the differential pressure sensor
is stored in memory. For determining the fill level, the meter fuel
is forced from the tube by pumping air in, and the fill level
(h(t)) is determined from a measure of the difference between the
instantaneous output signal (u(t)) and the memorized value
(U.sub.o).
Inventors: |
Jansche; Walter (Durmersheim,
DE), Zabler; Erich (Stutensee, DE), Dukart;
Anton (Maximiliansau, DE) |
Assignee: |
Robert Bosch GmbH (Stuttgart,
DE)
|
Family
ID: |
6380027 |
Appl.
No.: |
07/634,226 |
Filed: |
January 2, 1991 |
PCT
Filed: |
April 05, 1991 |
PCT No.: |
PCT/DE90/00265 |
371
Date: |
January 02, 1991 |
102(e)
Date: |
January 02, 1991 |
PCT
Pub. No.: |
WO90/13796 |
PCT
Pub. Date: |
November 15, 1990 |
Foreign Application Priority Data
Current U.S.
Class: |
73/301; 73/302;
73/714 |
Current CPC
Class: |
G01F
23/168 (20130101); G01F 23/18 (20130101) |
Current International
Class: |
G01F
23/18 (20060101); G01F 23/14 (20060101); G01F
23/16 (20060101); G01F 023/18 () |
Field of
Search: |
;73/299,301,302,714 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yasich; Daniel M.
Attorney, Agent or Firm: Greigg; Edwin E. Greigg; Ronald
E.
Claims
We claim:
1. A liquid container hydrostatic liquid level gauge, in particular
a fuel tank liquid level indicator, which has a differential
pressure sensor and a control and evaluation unit that determines
an instantaneous liquid level in said liquid container from
electrical output signals of the differential pressure sensor and a
display unit outside of said liquid container for displaying the
instantaneous liquid level, said liquid level indicator is disposed
in an interior of said container and includes a meter tube (24) of
small diameter, open at a face end juxtaposed a bottom of said
liquid container and extending from an upper end disposed above a
maximum liquid level (20') in said liquid container to juxtapose
the container bottom (121) and at an onset of measurement is filled
with liquid up to the liquid level of the container (20), said
differential pressure sensor (21) is provided with a first meter
input connection (211) to which said upper end of the meter tube
(24) is connected and a second meter input connection (212) which
is open to an air pressure (p.sub.0) which air pressure also acts
upon the surface of the liquid; a zero point displacement output
signal (U.sub.0) is stored in memory in the control and evaluation
unit (22) when the meter tube (24) is filled with liquid up to the
instantaneous liquid level, then an air pump (26) is connected to
an upper end portion of the meter tube (24) at a position above the
maximum liquid level, said air pump pumps air into said meter tube
(24) above the instantaneous liquid level which for liquid level
measurement forces out the liquid about said face end to evacuate
the meter tube (24), and that a liquid level determination in the
control and evaluation unit (22) is effected from a difference
between an output signal (u(t)) of the differential pressure sensor
(21) with the meter tube (24) evacuated and said stored zero point
displacement output signal.
2. A liquid container hydrostatic level gauge as defined by claim
1, in which a shutoff valve (27) is disposed in line with the air
pump (26) and is opened to fill the meter tube (24) with liquid up
to the instantaneous liquid level of the container and closed after
liquid is forced from the meter tube (24), and that the output
signal (u(t)) of the differential pressure sensor (21) associated
with the air filled meter tube (24) is picked up after closure of
the shutoff valve (27).
3. A liquid container hydrostatic level gauge as defined by claim
2, in which the differential pressure sensor (21) and the air pump
(26), including the shutoff valve (27) are disposed in the interior
of the container above the maximum liquid level (20').
4. A liquid container hydrostatic level gauge as defined by claim
2, in which the meter tube (24) has a widened portion (25)
extending over a short axial length at its lower end, the diameter
of which is large enough that a volume of the widened portion (25)
is large compared with a volume of the length of the meter tube
(24) above the widened portion.
5. A liquid container hydrostatic level gauge as defined by claim
4, in which the differential pressure sensor (21) and the air pump
(26), including the shutoff valve (27) are disposed in the interior
of the container above the maximum liquid level (20').
6. A liquid container hydrostatic level gauge as defined by claim
4, in which the liquid forced from the meter tube (24) is performed
only once at a beginning of a relatively long period of time after
a single pickup and memorizing of the output signal (U.sub.0) of
the differential pressure sensor (21) (zero calibration value) with
the meter tube (24) filled, and the fill level (h(t)) is derived
continuously from the continuous output signal (u(t)) over time of
the differential pressure sensor (21), which is reduced by the zero
point displacement output signal (U.sub.0).
7. A liquid container hydrostatic level gauge as defined by claim
6, in which the differential pressure sensor (21) and the air pump
(26), including the shutoff valve (27) are disposed in the interior
of the container above the maximum liquid level (20').
8. A liquid container hydrostatic level gauge as defined by claim
1, in which the differential pressure sensor (21) and the air pump
(26), including the shutoff valve (27), are disposed in the
interior of the container above the maximum liquid level (20').
9. A liquid container hydrostatic level gauge as defined by claim
1, in which in a course of the output signal of the differential
pressure sensor (21) during operation of the air pump over time is
monitored in the control and evaluation unit (22), and upon
recognition of an essentially constant amplitude with the air pump
operating over time, a control signal for the air pump (26) is
generated; said control and evaluation unit (22) is provided for
controlling the pump output which based on the control signal
reduces the pump output far enough that liquid is just barely
incapable of entering the evacuated meter tube (24), and that the
output signal (u(t)) of the differential pressure sensor (21)
associated with the evacuated meter tube (24) is picked up after a
reduction of the pump output.
10. A liquid container hydrostatic level gauge as defined by claim
9, in which the differential pressure sensor (21) and the air pump
(26), including the shutoff valve (27) are disposed in the interior
of the container above the maximum liquid level (20').
11. A liquid container hydrostatic level gauge as defined by claim
9, in which the evacuation of the meter tube (24) is performed only
once at a beginning of a relatively long period of time after a
single pickup and memorizing of the output signal (U.sub.0) of the
differential pressure sensor (21) (zero calibration value) with the
meter tube (24) flooded, and the fill level (h(t)) is derived
continuously from the continuous output signal (u(t)) over time of
the differential pressure sensor (21), which is reduced by the zero
calibration value (U.sub.0).
12. A liquid container hydrostatic level gauge as defined by claim
11, in which the differential pressure sensor (21) and the air pump
(26), including the shutoff valve (27) are disposed in the interior
of the container above the maximum liquid level (20').
13. A liquid container hydrostatic liquid level gauge in which a
circulation of liquid takes place via a liquid drain and a liquid
return, a meter tube (24') is disposed in the container interior
and is connected by its upper end to the liquid return (17, 19),
said meter tube (24') opens at a face end juxtaposed a bottom of
said container and extends from above a maximum liquid level (20')
as far as said container bottom (121); a differential pressure
sensor (21) communicates by a first meter input (212') with a lower
end of the meter tube (24') and by a second meter input connection
(211') is exposed to the liquid pressure at the container bottom
(121); an outflow opening (31) is provided at the lower end of the
meter tube (24'), with which a throttle (32) is associated, and an
overflow opening (30) is provided at an upper end of the meter tube
(24') above the maximum liquid level (20') if no circulation of
liquid takes place at an onset of measurement said meter tube (24')
is filled with liquid up to an instantaneous liquid level (20) and
a zero point displacement output signal (U.sub.0) is stored in
memory in a control and evaluation unit (22'), if circulation of
liquid takes place and a fill level determination is effected in
said control and evaluation unit (22') from a difference between
output signals (u(t)) of a differential pressure sensor (21) when
the meter tube (24') is filled with liquid up to the overflow
opening (30) and said stored zero point displacement output signal
U.sub.0).
14. A liquid container hydrostatic level gauge as defined by claim
13, in which said throttle (32) has a variable cross section.
15. A liquid container hydrostatic level gauge as defined by claim
13, in which the opening cross section of the throttle (32) is
dimensioned such that during liquid circulation the quantity of
liquid flowing to the meter tube (24') via the liquid (17, 19) per
unit of time is greater than the quantity of liquid flowing out of
the meter tube (24') by the outflow opening (31); and to obtain the
zero point displacement output signal (U.sub.0) of the differential
pressure sensor (21) when the meter tube (24') is filled, the
liquid circulation is temporarily suppressed.
16. A liquid container hydrostatic level gauge as defined by claim
15, in which the suppression of the liquid circulation is performed
only once at the beginning of a relatively long period of time, and
the zero point displacement output signal (U.sub.0) of the
differential pressure sensor (21) (zero calibration value) is
picked up and stored in memory, and that the fill level (h(t)) is
derived continuously, while liquid circulation is taking place and
with an operative throttle (32) from the continuous output signal
(u(t) over time of the differential pressure sensor (21), reduced
by the zero point displacement output signal (U.sub.0).
Description
PRIOR ART
The invention is based on a liquid container with a hydrostatic
level gauge, in particular a fuel tank with a tank level indicator,
of the type defined hereinafter.
There are many known versions of level gauges in automotive
engineering for monitoring the contents of the fuel tank. As fuel
tank shapes become increasingly complicated, the level gauge that
uses a float is increasingly being abandoned for more flexible,
nonmechanical measuring systems. For instance, electrothermal tank
level indicators or tank level indicators or level gauges operating
on the piezoelectric, acoustical or hydrostatic principle are
already known.
In hydrostatic level gauges, the hydrostatic pressure of the liquid
to be gauged is measured and from this a conclusion as to the level
is drawn by including the liquid density. To be able to measure the
hydrostatic pressure of the liquid in the interior of the
container, the differential pressure between the liquid pressure at
the bottom of the container and the air pressure above the surface
of the liquid is measured; a differential pressure sensor is
generally used for this. A substantial disadvantage of this
differential pressure method is the temperature dependency and the
long-term instability of the zero point in differential pressure
sensors available on the market.
ADVANTAGES OF THE INVENTION
The liquid container according to the invention having a
hydrostatic level gauge has the advantage that because of the
structural design according to the invention, a zero calibration of
the differential pressure sensor is readily possible and can be
performed at any time. For the zero calibration, the two
measurement inputs of the differential pressure sensors are exposed
to the same pressure. The electrical signal output by the
differential pressure sensor under these conditions is stored in
memory, and in the ensuing level measurement the output signal of
the differential pressure sensor is corrected upward or downward by
this memorized value, depending on the algebraic sign (+or -) of
this value. From the output signal, thus compensated for in terms
of zero point drift, the instantaneous level inside the container
is then determined by the evaluation unit, taking into account the
liquid density and the acceleration due to gravity. In this way,
not only is high measuring accuracy attained, but cost savings also
become possible, since zero point and aging stability are no longer
required of the differential pressure sensor and it can accordingly
be made at much less cost. Because the zero point stability is
dispensed with, more economical differential pressure sensors made
by thick-film technology can now be used.
Moreover, a predetermined disposition of the differential pressure
sensor on the container bottom is no longer compulsory; instead,
like the air pump it may also be disposed outside the container.
Only a very small tube is needed as a transmission route for the
hydrostatic pressure. The pump can be kept small, keeping the
technical expenditure low.
The freedom of liquid in the evacuated meter tube can be maintained
in various ways. In a first embodiment of the invention, a shutoff
valve is provided in line with the air pump; it opens to flood the
meter tube when the pump is shut off, and it is closed, with the
pump running, after evacuation of the meter tube; after that, the
pump is shut off again. Flooding of the meter tube may be done by
opening the shutoff valve first, prior to each measurement of the
level, or only at relatively long intervals, if a zero point drift
is suspected because environmental parameters (temperature) have
changed.
During these long intervals, the compression of the air column in
the meter tube (for example from acceleration of the liquid or from
temperature fluctuations) would cause liquid to rise in the meter
tube, making the outcome of measurement incorrect. This error is
kept small if in a preferred embodiment of the invention the lower
end of the meter tube is widened so extremely in a short axial end
segment that the volume of the widened portion is substantially
larger than the volume of the meter tube.
In a second embodiment of the invention, the evacuation of the
meter tube is maintained by providing that the pump stays in
operation continuously and maintains the pressure in the meter
tube. The shutoff valve can then be dispensed with. Since the
outcome of measurement would be made incorrect by the pressure drop
in the meter tube, which is dependent on the pump output, the
course of pressure over time in the meter tube, or in other words
the continuous output signal of the differential pressure sensor
over time, is monitored. Shortly after pump actuation, the output
signal has an approximately constant amplitude over time; that is,
the pressure in the meter tube is nearly constant. Now the pump
output is reduced far enough that just at that point no liquid can
rise in the meter tube. The pressure in the meter tube drops.
Because the pump output is now at a minimum, the pressure drop in
the meter tube leading to the incorrect result is also at a
minimum. The fill level can be derived directly from the electrical
signal output by the differential pressure sensor, after zero point
drift correction.
The liquid container according to the invention with a hydrostatic
level gauge has the same advantages as those given at the beginning
of this section and moreover makes it possible to dispense with the
air pump and shutoff valve. This lowers the production expense and
effort still further. However, this variant of the invention is
limited to liquid containers in which liquid circulation takes
place via a drain and return; hence it is preferably used as a
motor vehicle fuel tank having a tank level indicator, in which
fuel is aspirated from the fuel tank and excess fuel returns to the
fuel tank.
With interrupted liquid circulation, which is the case when the
vehicle engine is off, the meter tube is filled up to the level of
the instantaneous surface of the liquid in the interior of the
container, by the law of communicating tubes. The same pressure
acts upon the differential pressure sensor at both meter inputs.
The measured value output by the differential pressure sensor is
stored in memory as a correction value. When liquid is circulating,
in other words when the engine is running, the column of liquid in
the meter tube rises up to the overflow opening, since because of
the suitably selected cross section of the outflow opening at the
lower end of the meter tube, a larger volume of liquid per unit of
time flows to the meter tube than out of the meter tube. The
differential pressure sensor measures the differential pressure
between the hydrostatic pressure at the bottom of the meter tube
and that at the bottom of the container; the influence of the air
pressure above the surface of the liquid is eliminated, because
this air pressure has the same effect on the liquid in the meter
tube and on the liquid in the container. Since the height of the
overflow opening above the container bottom, the liquid density and
the acceleration due to gravity are known, the instantaneous fill
level can be readily ascertained from this differential
pressure.
If it is desired to dispense with interrupting the circulation of
liquid for the sake of zero point calibration, then in another
embodiment of the invention a valve of controllable cross section
should be provided at the outlet opening from the meter tube. For
the zero point calibration the cross section is opened far enough
that the volume of liquid flowing out via the cross section per
unit of time is larger than the volume of liquid flowing to the
meter tube. The height of the liquid column then matches the
surface of the liquid in the container interior. If the cross
section is narrowed again then, the level measurement can be
performed as soon as the liquid has risen far enough in the meter
tube that it emerges through the overflow opening. Once again the
zero point calibration can be performed here each time the fill
level is measured, or more practically, only at longer time
intervals. Between these times, the continuous output signal over
time of the differential pressure sensor, compensated for by the
zero point drift, is a direct measure of the instantaneous fill
level, so that a continuous indication that is only briefly
interrupted by a zero point calibration is available.
BRIEF DESCRIPTION OF THE DRAWING
The invention is described in further detail below in terms of
exemplary embodiments shown in the drawing. Shown are:
FIG. 1, a schematic diagram of a fuel tank with a hydrostatic tank
level indicator;
FIG. 2, a time-dependency diagram of the output signal of a
differential pressure sensor, in a tank level indicator modified
from FIG. 1;
FIG. 3, a diagram of the pump characteristic in the modified tank
level indicator;
FIG. 4, a schematic diagram of a fuel tank with a hydrostatic tank
level indicator in a further exemplary embodiment.
DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
FIG. 1, in the form of a schematic diagram, shows a fuel tank 10
with a tank level indicator 11, which serves as a general example
of a liquid container with a hydrostatic level gauge. The fuel tank
10 has a housing 12 with a filler neck 13 and tank cap 14. The
filler neck 13 is provided with a tank seal 15. The fuel intake
line 16 to the engine and the fuel return line 17 are connected to
the tank cap 14. The fuel intake line 16 discharges in an intake
neck 18, which extends to below the surface 20 of the liquid, near
the housing bottom 121, while the fuel return line 17 is connected
to a return neck 19 discharging above the liquid surface 20.
The tank level indicator 11, operating by the hydrostatic
principle, has a differential pressure sensor 21 with two meter
inputs 211 and 212, the pressure input 211 is subjected to a
pressure which the pressure of the fuel exerts at the bottom of the
tank, and input 212 is subjected to the air pressure within the
tank above the fuel level in the tank; a control and evaluation
unit 22 connected to the output 213 of the differential pressure
sensor 21; and a display unit 23. The tank level indicator 11 also
includes a meter tube 24 of small diameter, which is vertically
disposed in the housing 12 and extends from above the maximum
liquid level 20', which is represented by a dot-dash line, down to
the housing bottom 121. The end segment of the meter tube 24 is
flared extremely, over a very short axial length and open to the
fuel near the bottom of the tank, such that the volume of the
flared portion 25 of the meter tube is larger than the volume of
the meter tube 24. The meter input 211 of the differential pressure
sensor 21 and an air pump 26 are both connected to the upper end of
the meter tube 24. A shutoff valve 27 is in line with the air pump
26. The intake opening of the air pump 26, which can be opened or
closed via the shutoff valve 27, like the air pump 26 itself and
the shutoff valve 27, is located above the maximum liquid level 20'
in the housing 12 the pipe connection to the inlet of valve 27 is
within the tank above the fill line of the fuel. The meter input
212 of the differential pressure sensor 21 is also located above
the maximum liquid level 20', so that the differential pressure
sensor 21 is exposed on the one hand to the same air pressure that
acts on the surface of the liquid, and on the other to the pressure
in the meter tube 24. Both the opening and the closing of the
shutoff valve 27 and the switching on and off of the air pump 26
are controlled by the control and evaluation unit 22.
The mode of operation of the tank level indicator 11 is as
follows:
At the beginning of level measurement, the shutoff valve 27 is
first opened by the control and evaluation unit 22, with the air
pump 26 shut off. The meter tube 24 is thus flooded, causing the
meter tube 24 to fill with a liquid up to the height h, which is
equal to the height h of the surface of the liquid in the housing
12. The same pressure p.sub.0 that prevails in the portion of the
housing 12 not filled with fuel and that acts on the liquid surface
20 of the fuel 29 located in the housing 12 is also present at the
meter inputs 211 and 212 of the differential pressure sensor 21.
The output signal U.sub.O of the differential pressure sensor 21,
which is a measure for the zero point displacement or drift of the
differential pressure sensor 21, is stored in memory in the control
and evaluation unit 22.
The air pump 26 is now switched on by the control and evaluation
unit 22, with the shutoff valve 27 opened. The pump pumps air into
the meter tube 24 and forces the fuel located in it to return to
join the fuel quantity 29. After a predetermined period of time,
which is dimensioned such that the meter tube 24 is reliably free
of fuel, the control and evaluation unit 22 closes the shutoff
valve 27 and shuts off the air pump 26. The hydrostatic pressure of
the quantity of fuel 29 on the housing bottom 121 now acts upon the
meter input 211 of the differential pressure sensor 21. The output
signal u(t) of the differential pressure sensor 21 is continuously
present at the control and evaluation unit 22. From the difference
between the output signals u(t) and U.sub.0) of the differential
pressure sensor 21 with the evaluated and flooded meter tube 24,
the control and evaluation unit 22 determines the fill level h as a
function of time, in accordance with the following equation:
##EQU1## where g is the acceleration due to gravity, and .rho. is
the liquid density of the fuel. The continuous output signal over
time of the control and evaluation unit 22 is supplied to the
display unit 23, at which the fill level h(t) over an arbitrary
period of time can be read.
For the sake of measurement accuracy, the level measurement must be
interrupted from time to time and a new zero point calibration
performed; to this end, the meter tube 24 is flooded in the same
way by opening the shutoff valve 27, and the output signal U.sub.0
of the differential pressure sensor 21 is stored in memory with the
meter tue 24 flooded. In the level measurement ensuing after
evacuation of the meter tube 24, the new memorized value U.sub.0 is
now used for correcting the output signal u(t).
In the level gauge 11 in FIG. 1, the shutoff valve 27 can be
omitted, if the air pump 26 remains on for the duration of the
level measurement and thus prevents fuel from entering the meter
tube 24. To keep the measurement error as small as possible with
the air pump 26 running, the output signal u(t) of the differential
pressure sensor 21 is monitored, from the time of actuation of the
air pump 26 on. The course of this output u(t) over time is shown
in FIG. 2. Once the air pump 26 is switched on, with the meter tube
24 flooded, the pressure in the meter tube 24 initially rises and
then attains a maximum and after a brief time drops to an
approximately constant value. Once this value is attained, then the
pump output of the air pump 26 is throttled by the control and
evaluation unit precisely to such an extent that fuel is just
barely unable to enter the meter tube 24. With the reduction in
pump output, the flow of air through the meter tube 24, and thereby
the pressure drop at the meter tube 24 are also reduced. As can be
seen in FIGS. 2 and 3, the pump output is decreased at time t1; the
pressure in the meter tube 24 and correspondingly the amplitude of
the output signal u(t) of the differential pressure sensor 21
decrease accordingly. The output signal of the differential
pressure sensor 21 is a virtually error-free standard for the fill
level h in the fuel tank. The control and evaluation unit 22
calculates the fill level h(t) in the fuel tank 10 by equation 1,
and this is displayed in the display unit 23. In FIG. 3, the pump
characteristic of the air pump 26, that is, the pump pressure is
shown as a function of the required air quantity. At time t=t1, the
pump output is reduced, resulting in a parallel shift of the pump
characteristic to lower values.
In the fuel tank 10 in FIG. 4, the level gauge or tank level
indicator 11' is modified from the tank level indicator 11
described above. To the extent that components match those of FIG.
1, they have the same reference numerals. The meter tube 24', open
at the face end, disposed in the housing 12 again extends from
above the maximum liquid level 20' as far as the bottom 121, but it
is connected by its upper end to the return neck 19. The meter tube
24 also has an overflow opening 30 at its upper end and an outflow
opening 31 at its lower end, oriented toward the housing bottom
121; a throttle 32 is integrated with the outflow opening. The
throttle cross section is embodied such that the quantity of liquid
flowing to the meter tube 24' via the fuel return line 17 per unit
of time is larger than the quantity of fluid flowing out of the
meter tube 24' by the outflow opening 31 via the throttle cross
section. The lower end of the meter tube 24' is connected to the
meter input 212' of the differential pressure sensor 21, while the
meter input 211' of the differential pressure sensor 21 is exposed
to the hydrostatic pressure at the housing bottom 121. Once again,
the output 213 of the differential pressure sensor 21 is connected
to the control and evaluation unit 22', which in turn is connected
to the display unit 23.
The mode of operation of this modified tank level indicator 11 is
as follows:
When the vehicle engine is not running, no circulation of liquid
takes place; that is, no fuel is aspirated from the fuel tank 10
via the fuel intake line 16 and returned to the fuel tank 10 via
the fuel return line 17. By the law of communicating tubes, the
meter tube 24' is filled with fuel up to the liquid level 20. The
same hydrostatic pressure of the quantity of fluid 29 in the fuel
tank 10 is present at both meter inputs 211' and 212'. The output
signal U.sub.0 of the differential pressure sensor 21 that is
characteristic of the zero point deviation is stored in memory in
the control and evaluation unit 22'.
When the engine is running, fuel circulation takes place, and the
fuel flowing back to the housing 12 via the fuel return line 17
first flows into the meter tube 24'. Because of the cross section
of the throttle 32, dimensioned as described above, the fuel level
in the meter tube 24 rises until the overflow opening 30 is
reached, and the fuel flows through it to join the fuel quantity
29. Accordingly the meter tube 24' is always filled with fuel up to
the overflow opening 30. The overflow opening 30 is located at a
fixed distance 1.sub.0 from the housing bottom 121. The
differential pressure sensor 21 is now exposed on the one hand to
the hydrostatic pressure of the liquid quantity 29 at the fill
level h and on the other to the hydrostatic pressure of the fuel
column of height 1.sub.0 in the meter tube 24', The output signal
of the meter tube is thus proportional to the difference between
these two hydrostatic pressures. The output signal u(t) of the
differential pressure sensor 21 is supplied to the control and
evaluation unit 22', which from it calculates the output signal
h(t) in accordance with the following equation: ##EQU2## Once
again, g is the acceleration due to gravity, and .rho. is the
density of the fuel. U.sub.0 is the value, stored in memory and
characterizing the zero point drift, of the output signal of the
differential pressure sensor 21 when the vehicle engine is off. The
fill level, signal h(t) of the control and evaluation unit 22' is
shown in the display unit (23) and can be read out
continuously.
If it is desired to perform the zero point calibration of the
pressure sensor 21 independently of the shutoff of the vehicle
engine and the associated suppression of fuel circulation, then
instead of the throttle 32, a variable valve may be used and
connected to the outflow opening 31, a valve of variable cross
section may be provided. The control of the valve is effected via
the control and evaluation unit 22. For the zero point calibration
of the differential pressure sensor 21, in other words for
memorizing the output signal U.sub.0 of the differential pressure
sensor 21 while the meter tube 24' is flooded, the valve cross
section is opened far enough that the quantity of liquid that is
theoretically capable of flowing out of the meter tube 24' via the
outflow opening 31 per unit of time is larger than the quantity of
liquid flowing to the meter tube 24' via the return neck 19. With
the valve cross section dimensioned in this way, the fuel level in
the meter tube will adjust to the liquid level 20, so that the same
hydrostatic pressure is present at both meter inputs 211', 212' of
the differential pressure sensor 21. Once the output signal U.sub.0
of the differential pressure sensor 21 has been stored in memory in
the control and evaluation unit 21, the valve is triggered such
that its flow cross section is dimensioned like the throttle 32 in
FIG. 4, so that the quantity of fuel flowing into the meter tube
24' via the return neck 19 per unit of time is larger than the
quantity of fuel flowing out of the meter tube 24' via the valve in
the same unit of time.
* * * * *